The present invention relates to a liquid crystal display device of a type in which the alignment of a liquid crystal is controlled by applying to the liquid crystal layer electric fields that are parallel to the substrates between which the liquid crystal material is disposed. In particular, the invention relates to an active matrix liquid crystal display device that is improved in both viewing angle and display uniformity, a method for forming a photo-alignment layer for such a display device, and a method for checking the alignment ability of the photo-alignment layer.
Liquid crystal display devices are now widely used as devices for displaying various kinds of images, including a still image and a moving image.
Those liquid crystal display devices are basically classified into two types. In the first type, a liquid crystal panel is configured such that a liquid crystal layer is held between two substrates, at least one of which is made of transparent glass or the like, and pixels are turned on or off by selectively applying voltages to various pixel forming electrodes that are formed on the substrates of the liquid crystal panel. In the second type, various electrodes as mentioned above and pixel selecting active elements are formed, and pixels are turned on or off by effecting selection among the active elements.
In particular, the second type of liquid crystal display device, which is called an active matrix type device, is now the most used because it is superior in contrast performance, high-speed display performance, etc. Previous active matrix liquid crystal display devices have been of the so-called vertical electric field type in which electric fields for changing the alignment direction of the liquid crystal layer are generated in a direction transverse to the substrates between electrodes formed on one substrate and an electrode formed on the other substrate.
In recent years, liquid crystal display devices of the so-called lateral electric field type (also called the inplane switching or IPS type) have been realized in which the directions of the electric fields generated in the liquid crystal layer are approximately parallel with the substrate surfaces. Among liquid crystal display devices of this type is one in which a very wide viewing angle is obtained by forming comb-teeth-shaped electrodes on one of the two substrates (refer to Japanese Examined Patent Publication No. Sho. 63-21907 and U.S. Pat. No. 4,345,249).
On the other hand, a typical example of a method of aligning liquid crystal molecules of a liquid crystal layer in a predetermined direction provides an organic polymer thin film of, for instance, a polyimide type material, on a substrate and gives it an alignment ability by rubbing it. Such organic alignment layers have been put into practical use.
In another method (optical alignment method), alignment ability is imparted to an organic polymer thin film of, for instance, a polyimide type material formed on a substrate by illuminating it with light (refer to U.S. Pat. No. 4,974,941 and Japanese Unexamined Patent Publication Nos. Hei. 5-34699, Hei. 6-281937 and Hei. 7-247319).
However, these conventional optical alignment techniques have not been applied to the above-mentioned lateral electric field type display device. No consideration has been given heretofore to the remarkable effects that would be obtained when such optical alignment techniques are applied to the lateral electric field type display device, which has a different design concept than the vertical electric field type display device in which various pixel forming electrodes are formed on both substrates.
Although vertical electric field type active matrix liquid crystal display devices according to the conventional techniques can attain a wider viewing angle than twisted nematic type liquid crystal display devices, they have various problems, such as low display uniformity in large-size screens, the existence of a limit in increasing the viewing angle, and yellow or blue coloration in particular directions.
What is called the pre-tilt angle depends on the kind of polyimide-type polymer material used in an alignment layer. With a large pre-tilt angle, there is a problem that the viewing angle characteristic is not good and depends on the rubbing conditions.
Lateral electric field type devices, in which the alignment direction of liquid crystal molecules of a liquid crystal layer in the top substrate is the same as in the bottom substrate generally employ what is called a normally black scheme in which a black display is effected when no voltage is applied and a white display is effected when a voltage is applied. This scheme has a disadvantage in that display abnormalities due to display defects are more remarkable than in what is called a normally white scheme in which a white display is effected when no voltage is applied and which is employed in conventional twisted nematic type devices.
In the case of the homogeneous alignment of liquid crystal molecules, a slight alignment abnormality causes marked problems in the display.
The lateral electric field scheme has a disadvantage in that a slight variation in cell gap (interval between the two substrates of a liquid crystal panel, i.e., the thickness of a liquid crystal layer) causes a display non-uniformity, because the driving is performed according to the principle that the liquid crystal responds to the electric field rather than to a voltage.
On the other hand, the method of providing alignment ability by a rubbing treatment tends to cause static electricity on the film and to contaminate the film surface. Static electricity that is generated on an alignment layer by the rubbing treatment may destroy a thin-film transistor (TFT) operating as an active element or alter its switching characteristic. If an alignment layer is contaminated by the rubbing treatment, the frequency dependence of the threshold voltage of a pixel is made non-uniform. Further, since it becomes more difficult to properly control the load of the rubbing over the entire substrate area as the substrate size increases, the rubbing treatment may cause a scratch or unevenness in large-size substrates.
When rubbed with a cloth in a rubbing step, an alignment layer generates minute shavings and hence becomes a great source of dust in a clean room where liquid crystal display devices are manufactured, that is, a large factor in reducing yields of other related manufacturing steps. This is a serious problem.
Because of the existence of various pixel forming electrodes and asperities that result from the structure of active elements, such as TFTs, the substrate surface has height differences, which result in portions that are not rubbed in the rubbing step. Those portions may allow the passage of light during black display, which results in a contrast reduction.